Conventional direct-view field effect liquid crystal display (LCD) devices, such as twisted nematic (TN) displays and guest-host (GH) displays having dichroic dyes as the guest material in nematic or cholesteric hosts, are known. These conventional LCD devices have limited multiplex capacity because of a high V.sub.on /V.sub.th ratio, where V.sub.on and V.sub.th are the root-mean-square voltages for the device to be turned on and when the device is at threshold voltage, respectively. Furthermore, conventional nematic LCD devices have no storage effect, so that direct pel (picture element) drive or refresh circuits are necessary for their operation. For these reasons, in general, conventional nematic LCD devices are suitable only in display applications of the low information content type, such as digital watch displays.
To achieve greater versatility field effect LCD cells, such as the TN type, have been constructed. In such cells a liquid crystal material having a positive dielectric anisotropy is interposed between a pair of (upper and lower) parallel glass substrates, with the molecules of the LC material paralleled with the upper and lower substrates and twisted 90.degree. therebetween. Such an LCD cell is disposed between a pair of polarizers with polarizing axes intersecting with each other at right angles. With this typical construction, light impinging upon the device is first polarized linearly by one of the polarizers. Then its polarized plane is rotated 90.degree. by the liquid crystal molecules in a twisted arrangement and finally light transmits through the other polarizer. Where transparent electrodes formed with a pattern of a letter, digits, or other symbols are disposed on the inner surfaces of the upper and lower substrates, and are impressed with a voltage greater than the threshold voltage of the device, the LC (Liquid Crystal) molecules will be arranged in substantially the vertical direction or in the direction of the field. Under these conditions, the polarized plane of incident light will be intercepted by the analyzer. Thus the pattern can be displayed by controlling the direction of the LC medium within the LC cell to yield a transmission and an interception of the light.
Smectic liquid crystal display devices having a storage effect also known. This type of LCD device provides an indefinite storage of the information in the form of scattering regions in an otherwise clear background. More specifically, information is recorded by an intensity modulated laser beam which heats the LC material locally to create light-scattering centers. For more details see, for instance, "Laser-Addressed Liquid Crystal Projection Displays", by A. G. Dewey et al, pp. 1-7, Proceeding of the S.I.D., Vol. 19/1 (1978).
Optical storage effects in mixtures of nematic and cholesteric materials with negative dielectric anisotropy were observed and reported by Heilmeier and Goldmacher, Proceedings IEEE 57, 34 (1969). According to Heilmeier et al, a sample with no applied voltage was initially in a relatively clear state. The application of a DC or a low frequency AC voltage of a sufficient magnitude induced an intense scattering known as dynamic scattering. When the voltage was removed, the dynamic scattering disapppeared, but a quasi-permanent forward scattering stage remained. The storage decay time was reported to be on the order of hours. Furthermore, the scattering state could be erased and returned to the clear state by the application of an audio frequency signal.
The effects of weak boundary coupling on liquid crystal display performance is reported in an article by J. Nehring et al entitled, "Analysis of Weak-Boundary-Coupling Effects in Liquid-Crystal Displays" J. of Applied Physics 47, 850 (1976). According to the article, the multiplexing capacity of LCD devices can be improved by controlling the liquid crystal material-to-substrate anisotropy.
Heretofore several types of surface treatment techniques have been employed and applied in the making of liquid crystal display devices. For instance, in U.S. Pat. No. 4,140,371 entitled, "Liquid Crystal Display Devices", and issued to M. Kanazaki et al, an LCD device is described in which liquid crystals are oriented slightly inclined by the use of an orientation controlling structure formed by rubbing or oblique vapor deposition.
In order to control the alignment of the molecules of a liquid crystal material, some prior LCD devices employ surfactant coatings. For instance, in U.S. Pat. No. 3,967,883 entitled, "Liquid Crystal Devices of the Surface Aligned Type", and issued to D. Meyerhofer et al, it is described that one or more inside surfaces of an LCD device enclosure is coated with successive, slant-evaporated layers for the purpose of controlling the alignment of the molecules of the liquid crystal material.
Another prior LCD device employing the surface rubbing technique is described in U.S. Pat. No. 4,083,099 entitled, "Manufacture of a Twisted Nematic Field Effect Mode Liquid Crystal Display Cell", and issued to K. Yano et al. According to the patent, the surface of the transparent insulating film of the LCD device is rubbed to form micro-grooves aligned in a predetermined direction. The two glass substrates of the LCD device carry these transparent insulating films having micro-grooves formed using this rubbing technique. In addition, the application of this rubbing technique to promote the uniformity of the LCD optical effect is also mentioned and appreciated by M. Biermann, et al, in U.S. Pat. No. 3,892,471 entitled, "Electrodes for Liquid Crystal Components".
The effects of surface treatment on the liquid crystal material-to-substrate anisotropy is described in an article entitled, "Anisotropic Interactions Between MBBA and Surface-Treated Substrates", by S. Naemura, pp. C3-514-518, Journal De Physique, Colloque C3, supplement au no. 4, Tome 40. The article reported the measurements of the easy axis and anchoring strength coefficient between MBBA and substrates with various surfactants layers.
U.S. Pat. No. 4,028,692 of Ngo for "Liquid Crystal Display Device" requires no refresh because storage is provided.
U.S. Pat. No. 4,228,449 of Braatz for "Semiconductor Diode Array Liquid Crystal Device" describes an LC device incorporating semiconductor diodes with a storage mode.
U.S. Pat. No. 3,936,816 of Murata et al for "Flat Display System" shows a display system with row electrodes and column electrodes. The display elements can be LC devices, but no memory effect is described.
It will be seen from a careful consideration of all the foregoing art that there exists a need for an improved direct-view, high information content LCD device having convenient addressability along with an inherent storage effect.